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The EPS Industry Alliance (EPS-IA) recently completed a series of tests on extruded polystyrene (XPS) to examine the effects of moisture absorption and R-value in different field applications. Two new technical resources look at the behavior of rigid foam insulation exposed to water, specifically related to the materials drying potential and R-value retention.

When evaluating XPS material samples extracted from roofing and below grade applications, in these long-term installations, XPS did not maintain its initial R-value.

Buildings have been and always will be exposed to moisture. It is not a good thing or a bad thing; it is merely another component of the building design process. When materials are exposed to moisture, the ability to dry is key to maintaining thermal resistance.

This issue is addressed in Drying Potential of Polystyrene Insulations Under Extreme Environmental Cycling Conditions , which evaluates the free-thaw cycling effects on rigid foam plastics as prescribed by ASTM C1512. The test results indicate XPS exceeds the recommended water absorption threshold dictated by ASTM C578 by a factor of 2.4, and, test data rendered by Intertek Testing Services show that in-situ water absorption from XPS samples taken from four different locations is widely variable from 5 – 60% by volume.

Standardized laboratory testing, while not intended to replicate in-situ, real-world conditions, substantiate expanded polystyrene (EPS) performance claims to deliver consistent R-value in building environments that may be exposed to moisture. XPS producers claim its lower moisture absorption rate is a benefit; however, this is based on flawed logic. XPS R-values begin to deteriorate at only 0.03 percent, meaning its tolerance for water absorption is extremely low.

This phenomenon is demonstrated in the test results published in XPS Insulation Extracted After Field Exposure Confirms High Water Absorption Diminished R-value . On the other hand, EPS demonstrates excellent drying abilities and has a much higher tolerance for moisture exposure while still delivering the same R-value throughout the life of the building.

Expanded polystyrene exhibits superior moisture-related performance properties over XPS. It has higher vapor permeability, meaning it helps promote drying in a wall system. As shown in the EPS-IA technical bulletins, EPS is inherently more capable of tolerating moisture absorption than XPS.

When evaluating rigid foam insulation performance properties, non-standardized testing, modified test methods or testing not intended for the materials being evaluated should be viewed with skepticism. EPS-IAs new information comparing EPS and XPS moisture absorption and R-value retention is based on testing conducted by a third-party, certified testing laboratory and relies on industry recognized standards ASTM C1512. ASTM C518 and others.

EPS-IA is confident these new documents will be a valuable resource for architects, contractors and consumers that are seeking the best possible insulation for their construction projects. For more information on expanded polystyrene and the results of EPS-IAs new test results please contact Betsy Steiner, EPS-IA Executive Director, at emsteiner@epsindustry.org or 800-607-3772.

Contractors are increasingly called upon to install rigid foam insulation under concrete slabs and on building foundations.

Up to one-quarter of a building’s energy loss is due to lack of insulation in below-grade areas, including the foundation and under slabs. Now that high-performance building envelopes are common above ground, the relative amount of total heat lost below grade will grow if these spaces are not addressed.

As a result, superintendents increasingly will encounter below-grade and under-slab insulation on all building types. To help increase understanding of how two common rigid-foam insulations perform in these settings, this article evaluates moisture absorption and thermal performance. It also discusses installation procedures for below-grade and under-slab insulation.

An easy way to recognize EPS on the jobsite is that it is commonly white. This insulation is made of expanded polystyrene beads fused into sheet stock and blocks of various densities, compressive strengths and sizes. Historically used as a stable roof insulation, EPS has gained wide acceptance in wall, below-grade and under-slab applications due to its low-moisture absorption, strength and stable, long-term thermal performance. EPS insulation blocks can be custom-cut into a variety of shapes and sizes to meet wide ranging job specifications.

Building professionals have used EPS successfully in below-grade applications for decades. As of 2013, the International Code Council explicitly permits EPS throughout frost protected shallow foundations, under slabs and any other below-grade application.

XPS
To make XPS, manufacturers combine and melt polystyrene with blowing agents and additives, then force the liquid mixture through an extrusion die in a continuous feed, where it is shaped, cooled and trimmed to size. The product is most commonly available as boardstock of fixed size and thickness. Manufacturers often tint XPS a primary color for brand recognition.

EPS insulation absorbs significantly less moisture than does XPS insulation according to studies of real-world installations.

Moisture absorption and thermal performance
There is much confusion in the marketplace regarding whether EPS or XPS insulation resists moisture better. This is a key point, as wet insulation has lower thermal performance. While manufacturers of both insulation types tout that their products have lower moisture absorption, in-situ tests indicate that EPS performs better in this regard.

For example, in 2008, Stork Twin City Testing – an accredited independent testing laboratory – examined sheets of EPS and XPS removed from a side-by-side installation after 15 years in service on a below-grade foundation in St. Paul, Minnesota. The XPS was significantly wetter on extraction, with 18.9 percent moisture content by volume compared to 4.8 percent for the EPS. After 30 days of drying, the XPS still had elevated moisture of 15.7 percent, while the EPS had dried to 0.7 percent.

The U.S. Department of Energy’s Oak Ridge National Laboratory also reports high moisture absorption levels for XPS. In a 2012 study, the lab reported “all samples of XPS insulation gained much more moisture during the 15 years of contact with soil moisture.” The resulting loss of energy savings performance was 10 percent for a full basement (“deep basement”) and 44 percent for a slab-on-grade installation.

By comparison, the U.S. Army Cold Regions Research and Engineering Laboratory found EPS buried in wetted soil for 1,000 days absorbed only 1.7 percent moisture by volume, which is substantially lower than the XPS rates noted above.

Installing rigid foam insulation below grade
On building foundations, the insulation (whether EPS or XPS) is installed over the damp/waterproofing, after that layer has adequately cured. Crews can use mechanical fasteners or polystyrene-compatible adhesive to attach the insulation. Applying a bead of polystyrene-compatible caulk or mastic to the top of the insulation board minimizes water infiltration behind it.

For under-slab applications, the rigid foam insulation typically should be installed over a gravel base, with a poly vapor diffusion retarder between the gravel and insulation. Additional insulation is applied along the edges of the slab, because that is a primary surface for heat loss. To avoid damage to the insulation, it is necessary to ensure removal of any jagged surfaces or irregularities in the substrate before installing the rigid foam panels.

In either case, it is important to confirm all details with the insulation manufacturer and local building department, and to ensure appropriate construction techniques to drain water away from the building.

In addition to its lower moisture absorption and better long-term thermal performance, EPS has the highest R-value per dollar among rigid insulations. As such, it provides a cost-effective way to insulate building foundations, and under slabs.

Ram Mayilvahanan is the product marketing manager for Insulfoam, which offers below-grade insulation under the Insulfoam and R-Tech brand names. For more information, visit www.insulfoam.com.

Wet insulation is ineffective insulation – rigid foams that retain high volumes of moisture lose about half of their insulating R-value. Because insulation installed on below-grade building foundations and under concrete slabs is often exposed to moist soil, it is crucial to choose an insulation that has minimal long-term moisture retention and the ability to dry quickly.

For facility professionals that are evaluating insulation for building retrofits or for new construction, paying attention to moisture performance helps ensure effective long-term thermal resistance. Because the insulation will be hidden from view, any problems with degraded materials will not be obvious, although the effect on higher energy bills will be very real.

One challenge in selecting insulation is cutting through the competing claims of insulation manufacturers. Producers of extruded polystyrene (XPS) and expanded polystyrene (EPS) – common below grade insulations – both claim that their products are superior at resisting moisture. In their own ways, each one is right, but it depends on whether one is looking at abstract, standardized tests or performance in actual installed conditions.

Claims that XPS insulation absorbs less moisture than EPS are based on ASTM 272, Standard Test Method for Water Absorption of Core Materials for Sandwich Constructions. This test calls for fully submerging an insulation sample in water for 24 hours, then weighing it for moisture absorption immediately upon removal from the water.

How does this test represent reality? The truth is it doesn’t reflect real-world conditions for two reasons:

1) Unless your building is in a lake or river or subjected to severe flooding, the insulation will not be fully submerged.

2) It doesn’t account for how much an insulation dries out or does not dry out between periods of moisture exposure.

Entire marketing campaigns have been built around this test, but when it comes to what really happens on your building, it’s necessary to look at actual exposure during in-situ tests. Studies of insulation exposure to moisture in actual field conditions show that EPS outperforms XPS by a wide margin, largely because EPS dries much faster than XPS.

For example, the independent lab Stork Twin City Testing evaluated the moisture content of EPS and XPS buried side-by-side for 15 years on a building foundation in St. Paul, MN. At the time the insulations were removed, the EPS was four times drier than the XPS – the EPS had only 4.8% moisture by volume compared to 18.9% moisture content for the XPS. After 30 days of drying time, the EPS had dried to only 0.7% moisture by volume, while the XPS still contained 15.7% moisture.

The high moisture absorption of XPS is further seen in a 2012 report from the U.S. Department of Energy’s Oak Ridge National Laboratory. Researchers found that XPS insulation installed below grade for 15 years had absorbed 67% or more moisture. The resulting loss of energy savings performance for the XPS was 10% for a full basement (“deep basement”) and 44% for a slab-on-grade installation.

Insulation manufacturers are well aware of how their products will perform over the years. Evidence of this is seen in the limitations stated in warranties they offer. This is why XPS manufacturers typically warrant only 90% of the insulating R-value of their products during time in service, whereas most EPS manufacturers warrant 100% of the R-value. Some XPS manufacturers will also void warranties in case of ponding or water immersion, which runs contrary to their highlighting of 24-hour, full-immersion testing.

There are many claims in the market about whether EPS or XPS offers the best moisture resistance. When evaluating such statements, it is important to consider the basis upon which the statements are made. Does the testing involve guys in lab coats dunking insulation into a fish tank for one day, or does it replicate how insulation performs on actual buildings over many years? If facility managers are making the investment in insulation, this is an important distinction to pay attention to, otherwise the product might not perform as desired.

Which insulation is best for use on buried building foundations and under concrete slabs? Sales reps, naturally, will tell you that their company’s product is best. But, what does independent testing and research say?

These three tips will help your firm select a cost-effective and high-performance rigid foam insulation type for your next below-grade insulation job.

1. CONFIRM LONG-TERM THERMAL PERFORMANCE

Two of the rigid foam insulations most commonly used below grade and under slabs are expanded polystyrene (EPS) and extruded polystyrene (XPS). Although both are closed cell insulations, they perform very differently over the long term.

XPS has a higher initial insulating R-value than does a similar thickness and density of EPS, but the R-value of XPS degrades over time. EPS does not experience such “thermal drift,” and the reported R-value remains the same throughout years of installed service.

This is a crucial point when selecting insulation, as a decreasing R-value means lower thermal performance over time, and thus increased heating and cooling energy and costs for the building owner.

Rigid foam insulation is increasingly common in below-grade and under-slab applications.

A simple way to confirm an insulation’s long-term thermal performance is to review the warranty. Established EPS manufacturers typically warrant 100 percent of the published R-value for 20 years. By comparison, most XPS warranties typically cover only up to 90 percent of the published R-value, to account for the R-value degradation that occurs in the field.

2. ENSURE MINIMAL LONG-TERM MOISTURE ABSORPTION

In addition to R-value stability, rigid foam insulations differ in their rates of moisture absorption and their ability to dry. Wetted insulation provides lower thermal resistance and can degrade over time. Since insulation installed below grade frequently contacts wetted soil, rates of moisture absorption and the ability to dry is key in these applications.

Independent laboratories have conducted extensive tests of moisture absorption rates for both EPS and XPS. Although XPS often rates better in laboratory short term, fully submerged tests, real-world long term tests show that EPS performs much better. The reason is that EPS has the ability to dry much faster than XPS. This ability to dry at a fast rate helps EPS remain drier during conditions of repeated exposure to moisture.

A 15-year in-situ test of EPS and XPS dramatically demonstrated this point. Stork Twin City Testing evaluated the moisture content of EPS and XPS buried side-by-side for 15 years on a building foundation in St. Paul, Minnesota. At the time the insulations were removed, the EPS was much drier than the XPS—the EPS had only 4.8 percent moisture by volume, compared to 18.9 percent moisture content for the XPS. After 30 days of drying time, the EPS had only 0.7 percent moisture by volume, while the XPS still contained 15.7 percent moisture.

The high moisture absorption rate of XPS in real-world settings is further seen in a 2012 report from the U.S. Dept. of Energy’s Oak Ridge National Laboratory (ORNL). Their researchers found that XPS insulation installed below grade for 15 years had absorbed 67 percent or more moisture.

3. TARGET AN APPROPRIATE COMPRESSIVE STRENGTH

EPS insulation resists moisture better than XPS, including on buried foundations where it is in regular contact with wetted soil.

One of the best ways to save money on rigid foam insulation installed under concrete slabs is to ensure the material is not over-engineered. A common design assumption leads to specification of rigid foam strengths that are many orders of magnitude higher than necessary, which can double the insulation material cost.

Without getting into extensive technical details and mathematical formulas, the problem is engineers often use an overly conservative approach for insulation under concrete slabs. Many designers assume that point loads applied to a slab, such as those from the wheels of a forklift, transfer to the insulation in a triangular load path. Yet, concrete slabs distribute loads more uniformly than this, which means the insulation does not need as high of a compressive resistance as when one assumes a concentrated triangular load path.

An overly conservative design approach often results in specification of a high compressive resistance XPS product, when a more cost-effective EPS would offer sufficient strength. Since XPS typically costs more per inch than EPS, this is wasted money that comes off the contractor’s bottom line.

CONCLUSION

With building owners’ growing desire to save money on heating and cooling costs, and increasingly stringent energy codes, contractors will be installing below-grade and under-slab insulation on more of their projects. EPS insulation out-performs XPS for both long-term thermal resistance and long-term moisture absorption, and EPS comes in a variety of compressive strengths suitable for nearly all building projects. With the highest R-value per dollar, EPS is the cost effective insulation choice.

Useful references to support this article: NEW moisture absorption data regarding XPS, moisture absorption and the effects on R-Value was released in March 2014. Read more in the updated summary and in subsequent 2008 test program documents:

I am a retired architect, and may not have the best current information on EPS and XPS, but when these two products were mistakenly used as ‘flotation’ for lake docks and later removed, the XPS bales were like new and had no water soakage beyond the first (1/8 in.). However, I remember the EPS bales were waterlogged to the extent it took two people to even carry the bales. On top of this, the EPS bales showed a lot of disintegration due to freeze-thaw.

My observations may have been on EPS that had much less density (1-1/2 -2 #) than implied by The Construction Specifier article, but many reading will probably have the same concerns and begin to question the piece’s validity.

We asked the article’s author, Ram Mayilvahanan, to respond.

Mr. Bond raises a frequently discussed point about the long-term problems that arise when using rigid foam insulations that do not conform to ASTM standards.

Since insulation, especially below-grade, is out of sight, it can also be out of mind when it comes to ensuring the product being used at the job site matches the product that was specified. As with other building products, there are numerous companies making rigid foam insulations, often with varying degrees of quality. We building professionals share the responsibility in making sure the selected right-foam manufacturer can consistently deliver product that meets the specified performance.

To ensure performance on key factors, including moisture resistance, it is crucial to not only specify foam insulation that has been manufactured and tested to meet ASTM C578, Standard Specification fro Rigid, Cellular Polystyrene Thermal Insulation, but also to ensure the manufacturer supplying the foam insulation can consistently deliver quality product. A manufacturer’s longevity and track record with past projects should help in assessing this.

As an example, the floating green on the 14th hole in the world-famous Coeur d’Alene Golf resort in Idaho – considered on the of the coolest shots in golf- was built with EPS. It continues to be a testimony to well-engineered flotation insulation. Projects like this help establish the ability of manufacturers to deliver quality product.

Mr. Bond’s observation is a timely reminder for us building professionals that it pays to make sure the right product gets to the job site.

In the push to forge more energy-prudent builders, design professionals are leaving no part of the envelope unexamined. Walls and roofs have always presented a clear target for better thermal performance. Somewhat less obvious are surfaces that are out of sight – below-grade foundation walls and floor slabs. Well-engineered insulation in these locations can provide significant energy savings.

What separates below-grade insulation types from one another? Moisture retention, R-value stability, and compressive strength are the key performance attributes to consider when evaluating and comparing different below-grade insulations.

Installing thermal insulation on below-grade foundation or perimeter walls and under slabs is important because un-insulated concrete provides a thermal and moisture bridge between the heated building interior and the relatively cooler earth surrounding the building, or through exposed slab edges to the outside air.